Given enough inhabitants, any kind of organism can jeopardize the health of its ecosystem. The increasing pressure humans are placing on Earth is largely attributable to population growth. Farmable land, fresh water, energy, and transportation are in constant demand by all nations. Yet topsoil erosion, falling water tables, global warming, and air pollution are some of the many symptoms that suggest our planet's natural resources are overstressed by consumption.

Since 1900, world population has quadrupled from 1.5 billion to more than 6 billion people. The average number of children born to each woman has fallen since the 1960s, when global population reached its all-time peak growth rate of about 2 percent per year. Population growth in poorer, developing countries, however, continues to outpace growth in wealthier, developed countries. With more people in younger age groups, population growth in developing countries is expected to accelerate as today's children reach childbearing age.

Population growth alone does not tell the entire story. Humanity as a whole already uses more resources than nature can continually regenerate. Although the ecologically productive land available to each person on Earth is 1.7 hectares (4.2 acres), humanity's average "ecological footprint" measures 2.3 hectares (5.9 acres) -- some 35 percent greater than the available space. Developed nations consume more than their share of resources. As poorer nations attain prosperity and increase their use of resources, the picture becomes even bleaker.

An ecosystem that has been taxed of its resources or over-polluted loses its resilience. The maximum number of organisms that a given habitat's resources can support indefinitely is called its carrying capacity. Recent estimates of Earth's carrying capacity for humans range from one or two billion people living in prosperity to 33 billion people on minimum allowances.

Although advanced technologies like alternative energy sources or bioengineering may help solve some of society's present-day problems, many scientists, economists, and environmentalists are questioning whether the more populous developing countries like India and China should model Western industrial development as the means to improve their overall standard of living. They fear that by doing so, the increased consumption of natural resources and production of waste will stress the environment beyond its limits.

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